This invention relates generally to a treatment of water used in recirculating cooling water systems and more particularly to a water treatment composition which effectively controls corrosion and scale formation at elevated temperatures, which does not rely in any way upon chromate and which will allow for a reduction in the required levels of zinc and orthophosphate.
Corrosion occurs when metals are oxidized to their respective ions or insoluble salts. For example, corrosion of metallic iron can involve conversion to soluble iron in a +2 or +3 oxidation state or to formation of insoluble iron oxides and hydroxides. Metal loss from the solubilization of the iron causes the structural integrity of the system to deteriorate over time. Leakage between the water system and process streams can ultimately occur. Also, corrosion contributes to the formation of insoluble salts and the resultant buildup of deposits which impede heat transfer and fluid flow.
Chromate has traditionally been an extremely effective and widely used corrosion inhibitor. During recent years, however, the use of chromate has come under increasing scrutiny due to environmental concerns. In light of this, it is most desirable to develop chromate-free, environmentally acceptable corrosion inhibitors.
Additionally, a developing trend in cooling tower operation is toward higher recirculating water temperatures. The bulk water temperature in certain heat exchangers or reactor jackets reaches 170.degree.-180.degree. F. and does not fall below 130.degree. F., even in the cooling sections of the tower. Scale formation is more extensive at these higher temperatures since calcium and magnesium salts are typically less soluble. However, by reducing the salt concentrations that are added to the treatment composition, solubility will be enhanced. It is therefore also desirable to develop a chromate-free treatment program with lower levels of zinc and orthophosphate ions for corrosion inhibition at higher temperatures.